Electrochemically machined washer.
It may not look very impressive, but I'm quite pleased with this steel washer. The depression at the top was supposed to be a sharply-machined peace sign, created by electrochemical machining. Instead it's a blurry mess, but the point is that it's etched several millimeters deep into solid steel, and that was done using a cheap tin earring (shown under tin) as the machining bit.
The method is called electrochemical machining, and it's described in my July, 2009 Popular Science column.Source:Theodore GrayContributor:Theodore GrayAcquired:28 June, 2009Text Updated:29 October, 2009Price: DonatedSize: 2.5"Purity: >95%